It is often useful to suppress low spatial frequencies and enhance high spatial frequencies in an optical image produced by telescopes, cameras, radiometers and the like, in order to distinguish the high spatial frequency image elements from the low spatial frequency image elements. This allows the high spatial frequencies to stand out and become easier to locate, detect, and track so that point sources can be easily distinguished from a very similar background when the background is characterized by spatial frequencies which are lower than the higher frequencies of the point source.
Traditionally, the discrimination is performed electronically by massive data processing operations that require large computing capacity and a significant amount of time.
One attempt optically to preprocess images preliminarily to distinguish between objects or sources which differ from their background relates to the use of a dual beam interferometer which provides two images to a detector: a defocussed image and a sharply focussed image, whereby point sources may be detected. U.S. Pat. No. 4,128,337, Dec. 5, 1978, Method and Apparatus for Interferometric Background Suppression, Theodore F. Zehnpfennig.
It has further been suggested that techniques other than the focussing-defocussing approach may be used for discrimination purposes. In one proposal various techniques, such as introducing controlled amounts of spherical aberration, annular entrance apertures of various sizes, circular entrance apertures of various diameters, and transmittance variations, have been suggested to provide a spatial filter with two modulation transfer functions which match at lower spatial frequencies and diverge at higher spatial frequencies. To this end, pairs of optical systems were sought in which one member of the pair could be transformed into the other member and then back again with minimum mechanical disturbance to the instrument, using optical path difference variations and small oscillatory movements. It was found that one such pair could be formed by translating the central 30% portion of the primary mirror in a radiometer by one quarter wavelength to form one member, then the other member could be formed by removing the previous translation, translating the annular outer two percent of the primary mirror by one quarter wavelength, and finally oscillating the secondary mirror. See "Tailored Modulation Transfer Function and the Application to Dual Beam Interferometry", Scientific Report No. 1, Air Force Geophysics Laboratory, AFGL-TR-78-0077, Mar. 27, 1978, pages 1-29, Reference 1. Such an approach is difficult and costly to implement, and is subject to reliability and life problems because of the complexity of the mechanical and optical structures and interactions. Further, the treatment of a broad spectrum of input radiation has resulted in substantial mismatch of the various transfer functions which essentially defeats the matching at the low spatial frequencies and results in poor suppression.